Lab: building movies for the catalog

☕ Java 17 · Building Objects the Right Way with Constructors · 3/3

Goal: chain constructors with this() and super(), and watch the order in which their bodies run.

Setup: in an empty folder, create Content.java, Movie.java and StreamingPlatform.java.

Step 1 — The parent

public class Content {
    String title;

    Content(String t) {
        title = t;
        System.out.println("Content constructor: title set to " + title);
    }
}

The print shows exactly when this constructor runs.

Step 2 — The full constructor

public class Movie extends Content {
    private int minutes;

    public Movie(String t, int min) {
        super(t);
        minutes = min;
        System.out.println("Movie(String, int): runtime set to " + minutes + " minutes");
    }
}

super(t) hands the title to Content first.

Step 3 — A shortcut constructor

    public Movie(String t) {
        this(t, 120);
        System.out.println("Movie(String): default runtime applied");
    }

    public void play() {
        System.out.println("Now playing: " + title + " (" + minutes + " min)");
    }

this(t, 120) reuses the full constructor with a default runtime, so the logic lives in one place.

Step 4 — Build Dune

public class StreamingPlatform {
    public static void main(String[] args) {
        Movie dune = new Movie("Dune");
        dune.play();
    }
}

Predict the order, then run:

$ javac *.java
$ java StreamingPlatform
Content constructor: title set to Dune
Movie(String, int): runtime set to 120 minutes
Movie(String): default runtime applied
Now playing: Dune (120 min)

You called the one-argument constructor, yet Content prints first: Movie(String) → this(…) → Movie(String, int) → super(…) → Content(String). The bodies then finish from the top of the chain down.

Step 5 — Build Oppenheimer directly

Add to main:

        System.out.println("---");
        Movie oppenheimer = new Movie("Oppenheimer", 180);
        oppenheimer.play();

---
Content constructor: title set to Oppenheimer
Movie(String, int): runtime set to 180 minutes
Now playing: Oppenheimer (180 min)

Only two messages: the default-runtime step never ran. But the parent still came first. Whatever constructor you start from, super runs before the child's own code.

Full code on GitHub

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